What is a significant outcome of a cavity left untreated?

Choosing "contamination of the pulp" as the significant outcome of an untreated cavity is correct because a cavity, or carious lesion, progresses over time if not addressed. As the decay continues to erode through the enamel and dentin layers of the tooth, it can eventually invade the pulp chamber where the tooth's nerves and blood vessels are located. This invasion allows bacteria and toxins from the decaying material to enter the pulp, leading to pulpitis (inflammation of the pulp) and possibly resulting in an abscess or tooth death if the infection spreads.

The other outcomes, while they can occur as secondary effects of untreated cavities, do not represent the direct consequence of the cavity itself. For instance, tooth sensitivity can result from exposure of the dentin due to a cavity, but it is not the most immediate danger related to cavity progression. Extrusion of the tooth involves movement of the tooth within the socket, often due to gum disease or trauma rather than direct effects of a cavity. Cementum erosion may occur in various dental issues, but it is not a primary consequence specific to untreated cavities. Thus, the direct threat to the tooth and surrounding tissues from bacterial contamination of the pulp stands out as a critical and significant outcome.